Light energy is characterized by a dual nature both from a quantum point of view as photons and from a wave point of view as randomly polarized electromagnetic radiation with a wavelength between 400 nm and 700 nm. If the ultraviolet and infrared portion of the spectrum is included, the range of wavelengths is extended at both extremes. Presently, all practical solar cell energy collection schemes utilize the photon nature of light. For example, the conversion of solar energy to electrical energy using the photovoltaic effect depends upon the interaction of photons with energy equal to or greater than the band-gap of the rectifying material. With continued research, the maximum amount of energy captured using the photovoltaic mechanism is estimated to be around 30%.
Optical rectennas are known in the art for harvesting solar energy and converting it into electric power. Optical rectennas consist of an optical antenna to efficiently absorb the incident solar radiation and a high-frequency metal-insulator-metal (MIM) tunneling diode that rectifies the AC field across the antenna, providing DC power to an external load. The combination of a rectifying diode at the feedpoints of a receiving antenna is often referred to as a rectenna. Utilizing a rectenna to harvest solar energy relies upon the electromagnetic nature of radiation and is not limited by the band-gap of the rectifying material. As such, this method is not fundamentally band-gap limited. At microwave frequencies (˜2.4 GHz) the rectenna approach has been demonstrated to be approximately 90% efficient. Rather than generating electron-hole pairs as in the photovoltaic method, the electric field from an incident electromagnetic radiation source will induce a wave of accelerated electric charge in a conductor. Efficient collection of the incident radiation is then dependent upon resonance length scales and impedance matching of the collecting antenna to the rectifying diode to minimize losses. However, prior art methods of harvesting high-frequency radiation utilizing rectennas have identified several key problems with the approach. These problems include impedance matching, rectification, polarization, limited bandwidth and captured power.
Traditionally, the λ/2 dipole antenna is the most commonly used antenna by the designer as the receiving device for a rectenna due to the straightforward design procedure and the ease of fabrication as a printed circuit antenna. However, the λ/2 dipole has shortcoming as an antenna for an optical detector. A λ/2 dipole antenna only supports a single polarization. It exhibits a relatively low gain, it exhibits very high conductor losses at higher frequencies and its radiation pattern is omni-directional. It has been shown that the rectifier efficiency would be less than 0.1% for the calculated power at the terminal of a rectenna utilizing a λ/2 dipole antenna.
Polarization of solar radiation is known to be random (unpolarized). An unpolarized electromagnetic wave is a collection of waves that have an equal distribution of electric field orientations in all directions. A randomly polarized wave can be decomposed into two main components, Ex and Ey. The λ/2 dipole antenna as is commonly known in the art only supports a single polarization and is therefore not useful for the collection of solar radiation or other unpolarized electromagnetic energy.
Accordingly, what is needed in the art is an improved rectenna for the collection of electromagnetic energy and more particularly an improved rectenna for the collection of solar energy that overcomes the identified deficiencies in the prior art solutions.